This invention relates generally to radio systems wherein time-spaced, essentially monocycle-like signals are created from DC pulses and transmitted into space wherein the resulting energy bursts are dispersed in terms of frequency to where the spectral density essentially merges with ambient noise, and yet information relating to these bursts is recoverable.
Radio transmissions have heretofore been largely approached from the point of view of frequency channelling. Thus, coexistent orderly radio transmissions are permissible by means of assignment of different frequencies or frequency channels to different users, particularly as within the same geographic area. Essentially foreign to this concept is that of tolerating transmissions which are not frequency limited. While it would seem that the very notion of not limiting frequency response would create havoc with existing frequency denominated services, it has been previously suggested that such is not necessarily true, and that, at least theoretically, it is possible to have overlapping use of the radio spectrum. One suggested mode is that provided wherein very short (on the order of one nanosecond or less) radio pulses are applied to a broadband antenna which ideally would respond by transmitting short burst signals, typically comprising three or four polarity lobes, which comprise, energywise, signal energy over essentially the upper portion (above 100 megacycles) of the most frequently used radio frequency spectrum, that is, up to the mid-gigahertz region. A basic discussion of impulse effected radio transmission is contained in article entitled xe2x80x9cTime Domain Electromagnetics and Its Application,xe2x80x9d Proceedings of the IEEE, Volume 66, No. 3, March 1978. This article particularly suggests the employment of such technology for baseband radar, and ranges from 5 to 5,000 feet are suggested. As noted, this article appeared in 1978, and now, 16 years later, it is submitted that little has been accomplished by way of achieving commercial application of this technology.
From both a theoretical and an experimental examination of the art, it has become clear to the applicant that the lack of success has largely been due to several factors. One is that the extremely wide band of frequencies to be transmitted poses very substantial requirements on an antenna. Antennas are generally designed for limited frequency bandwidths, and traditionally when one made any substantial change in frequency, it became necessary to choose a different antenna or an antenna of different dimensions. This is not to say that broadband antennas do not, in general, exist; however, applicant has reviewed many types including bicone, horn, and log periodic types and has determined that none provided a practical antenna which will enable impulse radio and radar usage to spread beyond the laboratory. Of the problems experienced with prior art antennas, it is to be noted that log periodic antennas generally produce an undesired frequency dispersion. Further, in some instances, elements of a dipole type antenna may be configured wherein there is a DC path between elements, and such is not operable for employment in applicant""s transmitter.
A second problem which has plagued advocates of the employment of impulse or time domain technology for radio is that of effectively receiving and detecting the presence of the wide spectrum that a monocycle burst produces, particularly in the presence of high levels of existing ambient radiation, presently nearly everywhere. Ideally, a necessary antenna would essentially evenly reproduce the spectrum transmitted, and the receiver it feeds would have special properties which enable it to be utilized despite the typically high noise level with which it must compete. The state of the art prior to applicant""s entrance generally involved the employment of brute force detection, i.e., that of threshold or time threshold gate detection. Threshold detection simply enables passage of signals higher than a selected threshold level. The problem with this approach is obvious that if one transmits impulse generated signals which are of sufficient amplitude to rise above ambient signal levels, the existing radio services, producing the latter may be unacceptably interfered with. For some reason, perhaps because of bias produced by the wide spectrum of signal involved, e.g., from 50 mHz to on the order of 5 gHz or ever higher, the possibility of coherent detection has been thought impossible.
Accordingly, it is an object of this invention to provide an impulse or time domain (or baseband) transmission system which attacks all of the above problems and to provide a complete impulse time domain transmission system which, in applicant""s view, eliminates the known practical barriers to its employment, and, importantly, its employment for all important electromagnetic modes of radio, including communications, telemetry, navigation and radar.
With respect to the antenna problem, applicant has determined a truly pulse-responsive antenna which translated an applied DC impulse into essentially a monocycle. It is a dipole which is completely the reverse of the conventional bat wing antenna and wherein two triangular elements of the dipole are positioned with their bases closely adjacent but DC isolated. They are driven at near adjacent points on the bases bisected by a line between apexes of the two triangular elements. This bisecting line may mark a side or height dimension of the two triangular elements. Alternately, a monopole configuration is employed.
As a further consideration, power restraints in the past have been generally limited to the application of a few hundred volts of applied signal energy to the transmitting antenna. Where this is a problem, it may be overcome by a transmitter switch which is formed by a normally insulating crystalline structure, such as diamond material sandwiched between two metallic electrodes, which are then closely coupled to the elements of the antenna. This material is switched to a conductive, or less resistive, state by exciting it with an appropriate wavelength beam of light, ultraviolet in the case of diamond. In this manner, no metallic triggering communications line extends to the antenna which might otherwise pick up radiation and re-radiate it, adversely affecting signal coupling to the antenna and interfering with the signal radiated from it, both of which tend to prolong the length of a signal burst, a clearly adverse effect.
With respect to a radio receiver, a like receiving antenna is typically employed to that used for transmission as described above, although a single antenna and transmit-receive switch may Be substituted. Second, a locally generated, coordinately timed signal, to that of the transmitted signal, is either detected from the received signal, as in communications or telemetry, or received directly from the transmitter, as, for example, in the case of radar. Then, the coordinately timed signal, typically including a basic half cycle, or a few, up to 10 half cycles, of signal, is mixed or multiplied by a factor of 1 (as with sampling or gating of the received signals), or ideally, as where the coordinately locally generated signal is curved, the factor is greater than one, giving rise to amplification in the process of detection, a significant advantage. Thus, the modulation on a signal, or position of a target at a selected range, as the case may be, is determined. Such a detection is further effected by an integration of the detected signal, with enhanced detection being accomplished by both a short term (first) and long term (second) integration. In this latter process, individual pulse signals are, first, integrated only during their existence to accomplish short term integration, and following this, the resultant short term integration signals are long term integrated by integrating a selected number of these and particularly by a method which omits the noise signal content which occurs between individual pulse signals, thereby effecting a very significant increase in signal-to-noise ratio.
It is acknowledged that coherent detection of analog signals has been effected by the employment of coincidence detection, followed by only long term detection, but it is submitted that such coherent detection did not contemplate the local generation of a signal but contemplated storing of a portion of a transmitted signal which was then phase coordinated with the incoming signal, which on its face presents an essentially impossible task where there is the detection of a ultra wideband frequency pulse as in the present case.
Further, transmitted burst signals may be varied in time pattern (in addition to a modulation pattern for communications or telemetry). This greatly increases the security of the system and differentiates signals from nearly, if not all, ambient signals, that is, ambient signals which are not synchronous with transmitted burst signals. This also enables the employment of faster repetition rates with radar which would, absent such varying or dithering, create range ambiguities as between returns from successive transmission and therefore ranges. Burst signals are signals generated when a stepped, or near stepped, voltage change is applied to an impulse-responsive antenna as illustrated and discussed herein.
As still a further feature of this invention, the repetition rate of burst signals may be quite large, say, for example, up to 100 mHz, or higher, this enabling a very wide frequency dispersion; and thus for a given overall power level, the energy at any one frequency would be extremely small, thus effectively eliminating the problem of interference with existing radio frequency based services.
As still a further feature of this invention, moving targets are detected in terms of their velocity by means of the employment of a bandpass filter, following mixing and double integration of signals.
As a still further feature of the invention, when employed in this latter mode, two channels of reception are ideally employed wherein the incoming signal is multiplied by a selected range, or timed, locally generated signal in one channel, and mixing the same incoming signal by a slightly delayed, locally generated signal in another channel, delay being on the order of one quarter to one-half the time of a monocycle. This accomplishes target differentiation without employing a separate series of transmissions.
As still another feature of this invention, multiple radiators or receptors would be employed in an array wherein their combined effect would be in terms of like or varied-in-time of sensed (or transmitted) output, to thereby accent either a path normal to the face of the antenna or to effect a steered path offset to a normal path accomplished by selected signal delay paths.
As still another feature of this invention, radio antenna elements would be positioned in front of a reflector wherein the distance between the elements and reflector is in terms of the time of transmission from an element or elements to-reflector and back to element(s), typically up to about three inches, this being with tip-to-tip dimension of elements of somewhat below nine inches up to approximately nine inches.